Carbon Basics 1: What carbon is, and why it matters
What CO₂e means, how much CO₂ has built up in the air, how much of the 1.5 °C budget is left, and how big a tonne really is, shown in four 3D visuals instead of paragraphs.
This article is mostly written by Claude Code
Part 1 of "Carbon, from scratch". The series explains, from first principles, the work introduced on the Climate page.
Why this matters
The most unfamiliar part of reading carbon data is the units. Until you can read t CO₂e, ppm and Gt, you cannot tell a big number from a small one. The point of this post is to make those units understandable by visualizing them.
Carbon, CO₂, greenhouse gases, and CO₂e
Carbon (C), sixth in the periodic table, is an element; carbon dioxide (CO₂) is the gas it forms with oxygen. CO₂ is not the only gas heating the planet: methane, nitrous oxide and fluorinated gases do too. Because each gas warms differently, all of them are converted to a CO₂ basis so they can be expressed simply. That unit is CO₂e.
- CO₂74.4%
- Methane (CH₄)17.3%
- Nitrous oxide (N₂O)6.2%
- F-gases2.1%
| Gas | % |
|---|---|
| CO₂ | 74.4% |
| Methane (CH₄) | 17.3% |
| Nitrous oxide (N₂O) | 6.2% |
| F-gases | 2.1% |
Scale: concentration and the quota
CO₂ in the air is expressed as a concentration, in ppm: how many of every million air molecules are CO₂. Before industrialisation it was about 280 ppm, and it has risen every single year since measurements began in 1959.
| Year | ppm |
|---|---|
| 1960 | 316.9 |
| 1970 | 325.7 |
| 1980 | 338.8 |
| 1990 | 354.4 |
| 2000 | 369.7 |
| 2010 | 390.1 |
| 2020 | 414.2 |
| 2025 | 427.4 |
To stop global warming at 1.5 °C there is a fixed amount we can still emit. That is called the carbon budget. We have already used most of it, and if we carry on, carbon begets more carbon in a positive feedback loop. In other words, something out of the movie The Day After Tomorrow.
| Budget | Gt CO₂ |
|---|---|
| used | 2793.7 |
| remaining | 170 |
| about 4 years at the current rate | 42.2 Gt/yr |
Seeing what a tonne looks like
Most carbon numbers come in tonnes. Because CO₂ is a gas, a tonne takes up far more space than you would guess. Put a person and a car next to it and the scale clicks.
| Activity | kg CO₂e |
|---|---|
| One tree absorbs in a year | −2.35 kg CO₂e |
| Petrol car, 100 km | 16.2 kg CO₂e |
| 1 kg of beef | 60 kg CO₂e |
| 300 kWh of electricity (roughly a home for a month) | 125.2 kg CO₂e |
| One tonne | 1,000 kg CO₂e |
| World average, one person, one year | 4,729 kg CO₂e |
| Korea average, one person, one year | 11,286 kg CO₂e |
Why it matters
The budget is fixed, and to stay within it we have to be able to measure. That is why the carbon problem starts with measurement, and measurement starts with understanding the units correctly. In the next part we will look at where carbon is emitted in everyday life.
Sources and verification
- Atmospheric CO₂ annual means: NOAA Global Monitoring Laboratory, Mauna Loa (2025)
- Annual emissions and remaining carbon budget: Global Carbon Budget 2025
- Share by gas: Our World in Data (Climate Watch)
- Per-capita emissions: Our World in Data
- Korean figures (tree absorption, grid emission factor): cited under each scene
- CO₂ volume: density 1.87 kg/m³ at 15 °C and 1 atm